Void Requirements for Dense-Graded Bituminous Paving Mixtures

The void properties of compacted dense-graded bituminous paving mixtures are at least as important as their stability insofar as pavement performance is concerned. It is shown that the most significant void requirements for compacted bituminous mixtures are air voids and voids in the mineral aggregate (VMA). The values obtained for air voids and VMA for any given paving mixture depend upon whether the aggregate's ASTM bulk, ASTM apparent, or effective specific gravity has been employed to calculate them. For the design and control of dense-graded paving mixtures, it is recommended that requirements for air voids and VMA be based on the aggregate's ASTM bulk specific gravity, and that loss of bitumen by absorption into the internal capillary porosity of the individual aggregate particles must be allowed for when determining the air voids value. A useful chart for illustrating specification void requirements, and for assisting the design and control of bituminous paving mixtures, is described. By means of a graph, a tentative procedure is suggested for relating the mimimum VMA requirement of a compacted paving mixture to the nominal maximum particle size of the aggregate it contains. A general method for controlling the VMA value of a paving mixture by modifying the gradation of the aggregate is illustrated.